Take a forum with a large active membership, operated with a known proven support platform (vBulletin).
Allow the platform to fall into disrepair and poor performance by not investing in hardware and software maintenance and upgrades.
When the system threatens to die, hire an outside company to design a new independent forum platform. And don't get any input from known gurus in the industry on it. Tout it as the greatest thing since sliced bread. Try to force it on the membership.
When that bombs, quietly annnounce an "upgrade" back to vBulletin, but don't really upgrade anything.
Again allow the platform to fall into disrepair and poor performance by not investing in hardware and software maintenance and upgrades.
Once more, when the system begins to go into death throes, farm the forum operation out to another independent forum hosting service, offering only a small fraction of the functionality that the original software did. And in the meantime, make the transition for members as difficult as possible, by requiring re-registering, which requires the intervention of moderators or administrators, all of whom are far too busy to respond in any timely manner.
And all this while, do not keep the membership adequately informed on what is happening and why it is happening.
Have a nice day.